1. Field Of the Invention
The present invention is directed to method for operating a magnetic resonance imaging apparatus in accordance with a specified pulse sequence.
2. Description of the Prior Art
Short image pick-up times are of special significance in nuclear magnetic resonance tomography. This is true not only for obtaining an optimally high patient throughput but also, for example, in order to avoid motion artifacts. Short image pick-up times are indispensable for certain pick-up techniques, for example, cine mode (taking moving pictures).
Of methods currently known, the shortest image pick-up times (30 through 100 ms) can be achieved with the EPI method. The EPI method is described, for example, in European Patent 0 076 054.
In a typical EPI sequence, an examination subject is exposed to an RF excitation pulse in a first direction under the influence of a slice selection gradient at the beginning of the pulse sequence. Nuclear spins are thereby excited in a slice of the examination subject. After the excitation, a phase-coding gradient is activated in a second direction and a read-out gradient is activated in a third direction. The first, second and third directions reside perpendicularly relative to one another. The read-out gradient is composed of a pre-phasing pulse as well as of sub-pulses of alternating polarity. The nuclear spins are dephased and in turn rephased in alternation as a result of this alternating polarity of the read-out gradient, so that a sequence of nuclear magnetic resonance signals arises. So many signals are thereby acquired after a single excitation that the entire Fourier k-space is scanned, i.e. the existing data are adequate for reconstructing a complete tomogram.
The phase-coding gradient is briefly activated at every change in the polarity of the read-out gradient. The phase relation of the nuclear spins is thus advanced by one step every time.
The nuclear magnetic resonance signals that arise are sampled in the time domain, are digitized, and the numerical values acquired in this way are entered into a raw data matrix. An image of the examination subject is then reconstructed from this raw data matrix on the basis of a two-dimensional Fourier transformation.
The speed advantage of the EPI method arises essentially because a plurality of signals that are adequate for the reconstruction of a complete tomogram are acquired after a single excitation. All signals that ultimately represent gradient echoes must be acquired within the T2* decay. The read-out gradient must therefore be very rapidly bipolarly switched, so that considerable technological demands are made of the system, which constitutes a disadvantage of the EPI method.
In comparison to spin echoes, gradient echoes of the type generated in the EPI method also have the disadvantage of being sensitive to local field inhomogeneities.
German Patent 38 23 398, corresponding to U.S. Pat. No. 5,126,673, discloses a pulse sequence wherein a sequence of many equidistant RF pulses, referred to as a pulse burst, is used in order to excite a specimen. The RF pulses have an extremely small flip angle on the order of magnitude of 0.1.degree. through 2.degree.. A train of equidistant echo signals with optimally constant amplitude is obtained following upon the sequence of RF pulses. In order to keep the amplitude of the echo signals as constant as possible, the amplitude and phase of the RF pulses are influenced. A selected excitation refocusing, as well as read-out and phase-coding gradients are provided for the imaging.
The possibility is also mentioned of omitting a relatively large number of RF pulses from the equidistant sequence of RF pulses. According to the disclosure of the aforementioned patent, a minimization of the number of RF pulses can be achieved when the spacing between the RF pulses is varied corresponding to the sequence 1, 3, 5, 9, 17 . . . n, with n=2.sup.m-1 +1. The omission of RF pulses in the sequence of fundamentally equidistant RF pulses, however, is considered as being disadvantageous since the amplitude constancy of the echo signals is much more difficult to optimize than given a pulse burst composed of a gap-free sequence of RF pulses.
The non-equidistant radio frequency pulse sequence recited in the aforementioned patent also present the disadvantage that a number of echoes respectively coincide in the read-out phase, so that a clean evaluation for image acquisition is practically impossible. Moreover, the signal-to-noise ratio becomes extremely unfavorable in the excitation due to the small flip angles.